Optical fiber dispersion limits both the data rate and the length of an optical fiber link. Therefore, return-to-zero (RZ) differential quadrature phase-shift keying (DQPSK) is attractive as a modulation format for optical signals, because it provides superior tolerance to chromatic dispersion and polarization-mode dispersion (PMD).
A conventional RZ-DQPSK modulation system 100 includes a first dual-parallel Mach-Zehnder (DPMZ) modulator 110 and a second Mach-Zehnder (MZ) modulator 111 optically connected in series. The first DPMZ modulator 110 includes two MZ modulators 112 optically connected in parallel. The MZ modulators 112 of the first DPMZ modulator 110 are driven with non-return-to-zero (NRZ) electrical data signals 120 to generate NRZ optical data signals 130 having a relative phase shift of π/2, which are components of an NRZ-DQPSK optical data signal 131, by phase-modulating optical carrier signals 132. The second MZ modulator 111 is driven with an electrical clock signal 121 to generate an RZ-DQPSK optical data signal 133 by RZ pulse carving the NRZ-DQPSK optical data signal 131. Unfortunately, the use of a second MZ modulator 111 for RZ pulse carving in this conventional RZ-DQPSK modulation system 100 leads to increased manufacturing cost and decreased operating efficiency.
As an alternative to using a second MZ modulator for RZ pulse carving, an MZ modulator may be driven with an RZ electrical drive signal to generate an RZ optical data signal. As disclosed in U.S. Pat. No. 7,466,926 to Kao et al., issued on Dec. 16, 2008, which is incorporated herein by reference, an RZ electrical drive signal may be generated by applying a logic AND operation to an NRZ electrical data signal and an electrical clock signal to generate a first electrical intermediate signal, by applying a logic AND operation to an inverse of the NRZ electrical data signal and an electrical clock signal to generate a second electrical intermediate signal, and by differentially amplifying the first electrical intermediate signal and the second electrical intermediate signal to generate an RZ electrical drive signal. This approach to generating an RZ electrical drive signal requires the use of two high-speed logic gates.